1. Field of the Invention
The invention relates generally to reverse osmosis and ultrafiltration fluid separation processes, and is applicable particularly to relatively large scale apparatus for water desalination and purification by reverse osmosis, liquid waste treatment and food dewatering.
2. Prior Art
Desalination by reverse osmosis is achieved by pumping a feed stream of saline water at an elevated working pressure into a pressure resistant vessel containing an array of semi-permeable membranes. Purified product water of greatly reduced salinity permeates across the membranes into low pressure collection channels if the working pressure exceeds feed stream osmotic pressure. Considerable excess working pressure above the feed stream osmotic pressure is required to produce sufficient product water flux across membranes of reasonable surface area, and also to ensure sufficient dilution of the small but finite salt diffusion through the membrane which always exists when there is a concentration gradient across such membranes. For sea water whose osmotic pressure is about 25 Kg/sq. cm, typical working pressure for single stage reverse osmosis is in the order of 70 Kg/sq. cm.
While some of the feed stream permeates through the membranes, the balance becomes increasingly concentrated with salt rejected by the membranes. In a continuous reverse osmosis process, a concentrate stream must be exhausted from the vessel to prevent excessive salt accumulation. In sea water desalination, this concentrate stream may be typically 70% and sometimes as much as 90% of the feed stream. The concentrate stream leaves the vessel at almost full working pressure, but before the concentrate stream is exhausted from the apparatus, it must be depressurized. The concentrate stream is commonly depressurized by throttling over a suitable back pressure valve, for example a restrictor valve, which regulates the working pressure while dissipating all the pressure energy of the concentrate stream, but sometimes concentrate stream pressure energy is recovered using recovery turbine devices.
Furthermore, for high recovery concentration polarization must be controlled. Concentration polarization in the feed stream is the tendency for a concentration gradient to develop in the feed stream with high salt concentration on the membrane face during reverse osmosis. This tendency results from the bulk transport of saline feed water toward the membrane face and the accumulation of salt in the boundary layer as less saline water permeates through the membrane, balanced by diffusion of salt back out of the boundary layer. Concentration polarization is detrimental especially with feed solutions of high osmotic pressure such as sea water, because the membrane sees a higher concentration which raises the effective osmotic pressure. When concentration polarization occurs, working pressure for given product flux must be increased, product salinity will be increased, and membrane life may be impaired.
Reverse osmosis systems are typically designed to reduce concentration polarization effects by forced convection through the membrane array. Forced convection may be provided by circulating the feed fluid with a low ratio of product flow to concentrate flow through suitably configured feed channels between the membrane faces, or by auxiliary recirculation or by mechanical stirring devices. Operation at low ratios of product flow to concentrate flow is also generally favourable to the reduction of concentration polarization effects, but of course increases the feed pumping energy expenditure for a given product flow delivery.
Fouling by particulate or colloidal matter suspended in the feed water, by precipitates or by biological organisms is a severe problem in many potential industrial applications for reverse osmosis and ultrafiltration. Applications with a high risk of fouling have been generally handled using the tubular membrane configuration, in which the membranes line the bore of relatively large diameter and easily cleaned tubes. The tubular configuration is more bulky and costly than other membrane mounting systems.
Conventional single stage centrifugal pumps have been used as feed pumps to attain working pressures required for reverse osmosis. Large energy losses arise because a high speed and relatively large diameter impeller is revolving in a fixed casing. This results in high "disc friction" power losses arising from fluid drag between the casing and the impeller. Disc friction power loss is proportional to the cube of the relative velocity between impeller and casing, is almost independent of pump delivery flow, and is largely responsible for the low efficiency of small high head single stage pumps.
Energy transfer within a radial flow, high head centrifugal pump impeller is divided equally between static centrifugal pressure and kinetic energy of rotation. The diffuser accepts the high velocity fluid ejected from the impeller, and decelerates it to convert the kinetic energy into a further increment of pressure energy. Usually the efficiency of energy transfer in the impeller is much better than that of pressure recovery in the diffuser because of diffuser entrance and expansion losses. It is known to improve efficiency by providing an auxiliary rotor revolving intermediately between impeller and fixed casing but this increases complexity of redundant moving parts and the efficiency is decreased by transferring fluid from a moving diffuser to a stationary casing.
Reverse osmosis systems using rotary membrane pressure vessels are known, and take advantage of centrifuge action to reduce fouling of the membranes by suspended solids or colloidal matter and also to reduce concentration polarization. Some systems using external and physically separate feed pumps to pressurize the membrane rotor have serious practical and economic disadvantages of high pressure rotary seals and have low energy efficiency because of the use of conventional pump and turbine components.
Other known devices use rotary membrane pressure vessels which use centrifuge action to generate the entire working pressure required by the membranes as static centrifugal pressure. None of the kinetic energy of rotation is recovered as pressure energy, and indeed the kinetic energy imparted to permeated product fluid is lost when the permeate fluid is ejected from nozzles. Because relatively high tangential rotor speed is necessary to generate the working pressure, relatively large windage losses arise due to aerodynamic drag of a large high speed rotor. As the static centrifugal pressure rises quadratically with radial distance from the axis of rotation, only a thin annular volume within the membrane container vessel will practicably be available for active membranes as the pressure build-up rapidly becomes excessive moving outward from the radius where the necessary working pressure is first attained. In fact, apparatus in which all working pressure is generated by rotation of the membrane container vessel are limited realistically to relatively low pressure applications. The high working pressures required for reverse osmosis desalination of sea water and many other applications would result in an extremely high tangential rotor speed at the radius where the working pressure is attained. Such a high speed rotor, necessarily large to contain much useful membrane area, would have difficult stress and vibration design problems, and would not have high energy efficiency because of kinetic energy lost with the ejected permeate fluid and external windage losses.